This invention relates to a method of measuring concentricity of an optical fiber.
Referring to FIG. 1, an optical fiber is composed of a core 12 of high refractive index material surrounded by a cladding 14 of low refractive index material. In cross-section, the core and the cladding are substantially circular. Ideally, the core 12 and the cladding 14 are concentric, as shown in FIG. 1. However, current techniques for manufacture of optical fibers do not ensure that the core and the cladding are in fact concentric. An important measure of the quality of an optical fiber is the concentricity of the fiber, which is defined as the magnitude of the distance of the center of the core from the center of the cladding.
It is known to measure geometrical characteristics of an optical fiber by cleaving the fiber and employing a lens to image the end face of the optical fiber on a CCD sensor, which generates an electrical signal representative of intensity of illumination of the sensor as a function of position in the image plane by reference to a Cartesian coordinate system. This electrical signal constitutes an electronic image of the end face of the optical fiber and can be used to generate a video signal for driving a monitor to display a visually perceptible image of the end face of the fiber.
Using conventional image processing software, it is possible to analyze the electronic image generated by the CCD sensor and calculate the locations of a first group of points on the periphery of the image of the cladding and calculate the equation of a circle 16 (FIG. 2) that optimally fits the first group of points and calculate the locations of a second group of points on the periphery of the image of the core and calculate the equation of a second circle 18 that optimally fits the second group of points. It is then possible to calculate the distance xcex94X, xcex94Y along each axis of the coordinate system between the centers of the two circles. The magnitude of the distance D between the centers of the two circles is equal to
{square root over ((xcex94X2+L +xcex94=Y2+L ))}
If the imaging system were perfect, the magnitude of the distance D between the centers of the two circles, scaled to take account of the magnification of the imaging system, would be equal to the concentricity of the fiber. However, imperfections in the imaging system introduce an error such that in fact the image of the core is offset by an error vector E from the position 18xe2x80x2 that it would occupy if the imaging system were perfect and accordingly the true value Dxe2x80x2 of the distance between the centers of the two circles is equal to D-E. The concentricity of the fiber is equal to the magnitude of D-E.
It has been observed that if the fiber segment is rotated about the central axis of the cladding, the image of the center of the core describes a circular path and it can be shown that the center of the circular path is displaced from the image of the center of the cladding by a distance equal to the magnitude of the error vector (scaled to take account of the magnification of the imaging system). This observation has hitherto been used to demonstrate the existence of the error vector. However, to the inventor""s knowledge no other practical use has been made of this observation.
In accordance with the invention, it has now been recognized that the radius of the circular path described in the image of the center of the core when the fiber is rotated (scaled to take account of the magnification of the imaging system) is equal to the magnitude of the concentricity vector and that the concentricity of the fiber can be calculated-by measuring the radius of this circular path.
In accordance with a first aspect of the invention there is provided a method of determining concentricity of an optical fiber segment composed of a cladding and a core, both substantially circular in cross-section, the fiber segment having an end face and the core and the cladding having respective centers in the end face, and the fiber segment having a central axis extending through the center of the cladding, the method including positioning the fiber segment with the central axis of the fiber segment substantially coinciding with an axis of rotation, the fiber segment being in a first angular position about the axis of rotation, imaging the end face of the fiber on a detector, measuring the location of the image of the center of the core relative to the location of the image of the center of the cladding in the first angular position, rotating the fiber segment about the axis of rotation to a second angular position and measuring the location of the image of the center of the core relative to the location of the image of the center of the cladding in the second angular position, rotating the fiber segment about the axis of rotation to a third angular position and measuring the location of the image of the center of the core relative to the image of the center of the cladding in the third angular position, and calculating the radius of the circle that circumscribes the locations of the image of the center of the core relative to the image of the center of the cladding.
In accordance with a second aspect of the invention there is provided a method of determining concentricity of an optical fiber segment composed of a cladding and a core, both substantially circular in cross-section, the fiber segment having an end face and the core and the cladding having respective centers in the end face, and the fiber segment having a central axis extending through the center of the cladding, the method including (a) positioning the fiber segment with the central axis of the fiber segment substantially coinciding with an axis of rotation, the fiber segment being in a first angular position about the axis of rotation, (b) employing an imaging system to create an image of the end face of the fiber, (c) measuring the location of the image of the center of the core relative to the location of the image of the center of the cladding in the first angular position, (d) effecting relative rotation of the fiber segment and at least one element of the imaging system about the axis of rotation to a second angular position and measuring the location of the image of the center of the core relative to the location of the image of the center of the cladding in the second angular position, (e) effecting relative rotation of the fiber segment and said at least one element of the imaging system about the axis of rotation to a third angular position and measuring the location of,the image of the center of the core relative to the image of the center of the cladding in the third angular position, and (f) calculating the radius of the circle that circumscribes the locations of the image of the center of the core relative to the image of the center of the cladding.